A winding in a motor is equivalently made by an inductance and a resistor, so that disconnection can be detected on the basis of current flowing when a DC (Direct Current) voltage is applied. There is a motor disconnection detection system for alternately applying positive and negative voltages in predetermined cycles to a motor connected to an H bridge circuit and performing disconnection detection on the basis of the number of pulses per unit time of a pulse signal obtained by differentiating a current signal detected by a motor current detecting means (refer to Patent Document 1).                Patent Document 1: JP-2002-238291 A        
On the other hand, there is a load, such as an antenna and specific sensor, in which a capacitor is formed in series equivalently between terminals. Disconnection of such a load cannot be detected in a DC driven state. FIG. 5 shows a conventional disconnection detecting circuit provided for a drive circuit for an antenna. A drive circuit 2 for an antenna 1 is a so-called H-bridge circuit including, between power supply lines 3 and 4, (i) N-channel-type MOSFETs 7 and 8 sandwiching an output terminal 5 and connected in series and (ii) N-channel-type MOSFETs 9 and 10 sandwiching an output terminal 6 and connected in series. By alternately turning on/off the MOSFETs 7 and 10 and the MOSFETs 8 and 9 in accordance with the output frequency, alternate current is passed to the antenna 1.
To the MOSFET 10, a series circuit of a MOSFET 11 for sensing current and a shunt resistor 12 is connected in parallel. A comparator 13 compares a voltage Vc applied across terminals of the shunt resistor 12 with a reference voltage Vref, thereby obtaining a disconnection detection signal Sb. FIGS. 6A and 6B show wavelengths when the antenna 1 is normally connected, of the voltage of the output terminal 5, current flowing in the antenna 1, the voltage across the terminals of the shunt resistor 12, and the disconnection detection signal Sb in order from the top. FIG. 6A shows a desired resonance state where the antenna 1 and the drive circuit 2 match. FIG. 6B shows a state deviated from the desired resonance state due to the kind of the antenna, the constant of the antenna, variations in the constant, and the like.
When the center position of the on period of the MOSFETs 7 and 10 is set as a predetermined detection timing P, in the case of FIG. 6A, the disconnection detection signal Sb becomes the L level when the antenna 1 is disconnected, and the disconnection detection signal Sb becomes the H level when the antenna 1 is connected. Consequently, disconnection can be normally detected. In contrast, in the case of FIG. 6B, due to delay of the phase of current, also when the antenna 1 is connected, the voltage across the terminals of the shunt resistor 12 at the detection timing P drops, the disconnection detection signal Sb becomes the L level, and erroneous detection occurs. Depending on the deviation of resonance, the phase of current advances.
To prevent the erroneous detection, a method of setting the reference voltage Vref to be low may be considered. However, when the reference voltage Vref is decreased, erroneous detection due to noise tends to occur. A method of increasing current flowing in the shunt resistor 12 by increasing the size of the MOSFET 11 for sensing current may be also considered. However, the chip area in an IC increases, and the cost increases.